Biosynthesis of aromatic monoterpenes: Conversion of γ-terpinene to p-cymene and thymol in Thymus vulgaris L

Abstract

The major components of the volatile oil of thyme ( Thymus vulgaris L.) are the aromatic monoterpenes, thymol (38%) and p-cymene (23%), and the cyclic diene, γ-terpinene (28%). The time course of incorporation of 14CO 2 into the volatile terpenoids of thyme cuttings suggested a biosynthetic sequence by which γ-terpinene gave rise to p-cymene, which in turn yielded thymol. Intact leaves from young thyme plants converted exogenous γ-[G- 3H] terpinene into p-cymene and thymol, whereas p-[G- 3H]cymene gave rise to only thymol in this tissue. Preincubation of the tissue with unlabeled p-cymene followed by incubation with γ-[G- 3H]terpinene resulted in an increase of radioactivity in p-cymene and a substantial decrease of radioactivity in thymol. Similar isotopic dilution experiments with other possible intermediates, such as α-terpinene and terpinen-4-ol, produced no significant change in product distribution. All of the biosynthetic products were identified by radio gas-liquid chromatography and by the synthesis of several derivatives, which were analyzed by radio chromatography or crystallized to constant specific activity. These results provide, for the first time, strong evidence that thymol is biosynthesized by the aromatization of γ-terpinene to p-cymene followed by hydroxylation of p-cymene.